The present invention relates to an electronic timepiece having a star display.
There has been proposed an electronic wristwatch having a star map on a star map disk which is rotatably mounted under a dial of the watch. The dial has an opening through which a part of the star map is disclosed to display constellation figures. The disk is simultaneously driven with hour, minute and second hands by a single power source and is adapted to rotate one revolution per one sidereal day.
In such a watch, if an additional function for discerning twilight is provided, it would be very useful not only for astronomical observation, but also for usual outdoor activity, since outdoor activity is possible in the twilight.
However, it is difficult to provide a device which discerns the twilight accurately.
The interval of time, during which the sun is between the horizon and 6.5 degrees below the horizon, is called a civil twilight. The interval of time, during which the sun is between 12 and 18 degrees below the horizon, is called an astronomical twilight. In the civil twilight, the brightest planet can be observed.
In order to discern twilight conditions, an accurate solar position in the celestial sphere (right ascension and declination), the latitude of the observation point, and the local sidereal time are necessary. However, a moving rate of the sun in the ecliptic in the celestial sphere is not constant because the revolution orbit of the earth is an ellipse. Further, the ecliptic crosses the celestial equator at an angle of about 23.5 degrees and the declination of the sun has seasonal variation. Therefore, accurate solar position cannot be indicated, for example, by means of a hand uniformly rotating one rotation per day.
Further, in the prior art, stars and constellation figures in a northern part of the northern celestial hemisphere, namely circumpolar stars and constellations around the stars, are not included. Accordingly, such a watch does not meet requirements of users.